Tetrahydrobiopterin is a required cofactor for the hepatic phenylalanine hydroxylating system. De novo synthesis of tetrahydrobiopterin is regulated mainly by the activity of the first enzyme of the biosynthetic pathway, GTP cyclohydrolase I (GTPCH). The GTPCH feedback regulatory protein (GFRP) forms a complex with GTPCH and confers endproduct feedback sensitivity. GFRP mRNA distribution in rat brain has now been determined and correlated with GTPCH mRNA levels. All aminergic neurons contain both GFRP and GTPCH mRNAs, although the ratio of GFRP to GTPCH mRNA is much greater in serotonergic neurons than in dopaminergic neurons, where it regulates tetrahydrobiopterin and serotonin synthesis. Tetrahydrobiopterin is also required by all forms of nitric oxide synthase and limits production of nitric oxide, endothelial derived relaxing factor, by endothelial cells and vascular smooth muscle cells. Availability of intracellular tetrahydrobiopterin likely plays a role in controlling nitric oxide-dependent vascular tone under normal and pathological conditions. Sphingolipid metabolites, ceramide, spingosine, and sphingsosine-1-phosphate, play important signaling roles in many biological processes, including cell survival and death. We have now found that although ceramide mediates some of the actions of TNF-alpha in the induction of nitric oxide synthase and nitric oxide production in glial cells, TNF-alpha stimulates GTP cyclohydrolase activity and tetrahydrobiopterin synthesis by a ceramide independent mechanism. This is the first indication that these two pathways may be regulated via distinct signaling pathways and by different mechanisms. - tetrahydrobiopterin nitric oxide phenylketonuria athersclerosis sphingolipids